1. Field of the Invention
This invention relates to the field of high solids thermosetting coating compositions.
2. Description of the Prior Art
Sekmakas, U.S. Pat. No. 4,144,220, teaches essentially solvent-free resinous coating compositions which thermoset to provide chemically resistant coating compositions and are made by copolymerizing monoethylenically unsaturated monomers including monomers having a functional group selected from carboxylic acid, hydroxy and amide groups in a solvent medium consisting essentially of liquid water insoluble trihydric alcohol having a molecular weight of at least 500, and then adding an aminoplast resin. The ratio of reactive copolymers to trihydric alcohol is from 0.15 to 0.80 in the Sekmakas patent. Sekmakas does not teach soft, low molecular weight acrylic copolymers having a combination of acid and hydroxy functionality.
Fry et al, U.S. Pat. No. 3,267,174 teach high solids acrylic coating compositions prepared by thermoset crosslinking acrylic acid copolymer, polyalkylene polyol and a formaldehyde-derived resin. The acrylic acid copolymer must have a glass transition temperature, T.sub.g, of at least 25.degree. C.
The high solids formulations which Fry et al teach are very viscous solutions which would not be suitable for spray, dip, or roller coating applications. Furthermore, Fry et al do not teach acrylic acid copolymers which also contain hydroxy functionality.
Smith U.S. Pat. No. 4,256,522 discloses a four component composition comprising a non-aminoplast hydroxyl bearing polymer, (poly)glycol monoether, an aminoplast or phenolic resin capable of curing the hydroxyl bearing polymer, and a strong acid catalyst for the reaction between the first and third components. The (poly)glycol monoether is a monohydroxy material and, as such, is incapable of crosslinking and, in fact, the teachings of the patent imply that the monoether does not enter into the reaction.
Smith U.S. Pat. No. 4,351,928 is similar to the aforementioned Smith patent except rather than the monother, a liquid amino- or hydroxy aromatic diluent is used. In this diluent the hydroxy group(s) are attached to the aromatic ring.
Chang et al U.S. Pat. No. 4,343,025 teaches liquid coating compositions consisting essentially of a curable film-forming polyol-containing component and an organic reactive diluent which contains one primary or secondary hydroxyl group. The coating compositions can be formulated to contain little or no volatile organic solvent and/or water.
The reactive diluent approach to meeting air quality environmental requirements has been suggested by others. "Proceedings of the ninth Water-Borne and Higher Solids Coatings Symposium, Feb. 17-19, 1982," pages 231-251, is exemplary of the state of the art. In this paper the authors found little benefit to the reactive diluent approach to increasing the nonvolatiles in high solids coatings.
In another paper exemplary state of the art, High Solids Coatings, March 1982, pages 17-18 entitled "A Critical Evaluation of High solids Coatings," Sharma, also discusses disadvantages in high solids coatings--the discrepancy between theoretical and actual volume solids.